(1) Field of the Invention
The present invention generally relates to a bag for an air bag system for a vehicle and a method of manufacturing the same and, more particularly, to a bag for an air bag system which has an auxiliary bag for controlling the shape of a main bag during inflation of the main bag.
(2) Description of the Related Art
In the field of automobile technology, an air bag system is well known as a system for restraining a driver or a passenger. The air bag system inflates a bag between the driver and the steering wheel when a deceleration exceeding a given threshold level is exerted on the automobile. Thus, a body of a driver or a passenger is restrained when a automobile is abruptly decelerated.
It is required that the bag be suitably inflated to restrain the driver without any failure. Especially, shapes of the bag during the inflation thereof are very important to control movements of the driver appropriately.
Therefore, a bag which has an auxiliary bag inflating at the end of the inflation of the main bag has been used in a conventional air bag system as disclosed in Japanese Laid-open Patent Application No. 3-7748.
The bag disclosed in the above application has an underside base cloth and an upperside base cloth. The underside base cloth has an inlet for gas supplied from an inflater placed at the center of a steering wheel. The upperside base cloth has auxiliary bags made from sections of the upperside base cloth.
The auxiliary bags are turned into the reverse side of the upperside base cloth. More particularly, the auxiliary bags are turned in so that they start their inflation when stresses exerted thereon increase to a predetermined level which occurs at the end of the inflation of the main bag. Thus, the bag is inflated with a volume not including the volume of the auxiliary bags at the beginning of the inflation. Thereafter, it is inflated as a bag which has a volume including the volume of the auxiliary bags.
That is, the size of the bag is sequentially changed during the two levels of the inflation process. As a result, it is possible to realize both arranging the shape of the bag appropriately in the beginning of the inflation and securing a sufficient volume for restraining the driver without any failure at the end of the inflation.
The conventional bag is manufactured as follows. First, auxiliary bags are made by turning sections of the upperside base cloth into the reverse side thereof and stitching the upperside base cloth at the base of the sections so that the stitched portions have an appropriate strength. Then, the upperside base cloth and the underside base cloth are stacked so that the reverse sides thereof are in contact with each other. Thereafter, main bag is made by stitching the upperside base cloth and underside base cloth together along their periphery.
In the above manufacturing method, it is necessary to form the auxiliary bags before stitching the upperside base cloth and the underside base cloth together. This is because the auxiliary bags are made from sections of the upperside base cloth which should be located interiorly of the main bag. Thus, in the conventional method, it is not possible to form the main bag and the auxiliary bag at the same time.